In recent years, the development of power storage devices such as a lithium ion secondary battery and a lithium ion capacitor has been conducted. Of secondary batteries, a lithium ion battery formed using a metal oxide containing lithium, such as lithium iron phosphate has high capacitance and high safety.
As typical structure examples of lithium iron phosphate that is used for a positive electrode of a secondary battery, there are LiFePO4 of an olivine structure that contains a divalent iron ion (hereinafter referred to as Fe(II)) and Li3Fe2(PO4)3 of a Na Super Ionic Conductor (NASICON) structure that contains a trivalent iron ion (hereinafter referred to as Fe(III)). LiFePO4 of an olivine structure that contains Fe(II) is an excellent positive electrode material which has high capacitance and a high discharge potential (see Patent Document 1).
In a battery in which lithium iron phosphate is used as an active material of a positive electrode, a redox reaction occurs in iron due to insertion and extraction of a lithium ion. For example, it is known that in LiFePO4 having an olivine structure, a redox reaction between Fe(II) and Fe(III) occurs due to insertion and extraction of a lithium ion. In a similar way, in lithium iron phosphate containing Fe(III), such as Li3Fe2(PO4)3 having a NASICON structure, redox reaction between Fe(III) and Fe(II) occurs (see Patent Document 2).
A thin-film battery whose electrode is formed by a technique for forming a thin film (e.g., a sputtering method) has characteristics such as excellent flexibility and high durability and has attracted attention in recent years. In many cases, lithium iron phosphate formed by a sputtering method has an amorphous structure. Moreover, when subjected to heat treatment, lithium iron phosphate having an amorphous structure can be changed into lithium iron phosphate having a crystalline structure, such as LiFePO4 having an olivine structure or Li3Fe2(PO4)3 having a NASICON structure. Lithium iron phosphate having an amorphous structure and lithium iron phosphate having a crystalline structure can be used as an active material of a positive electrode.